Catalyst for hydrogen generation through steam reforming of hydrocarbons

ABSTRACT

A catalyst that can be used for the production of hydrogen from hydrocarbon fuels in steam reforming processes contains an active metal of, e.g., at least one of Ir, Pt and Pd, on a catalyst support of, e.g., at least one of monoclinic zirconia and an alkaline-earth metal hexaaluminate. The catalyst exhibits improved activity, stability in both air and reducing atmospheres, and sulfur tolerance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to catalysts. In particular, the presentinvention relates to catalysts that can be used for the production ofhydrogen from hydrocarbon fuels.

2. Discussion of the Background

Hydrogen production from natural gas, propane, liquefied petroleum gas,alcohols, naphtha and other hydrocarbon fuels is an important industrialactivity. Hydrogen is used industrially in the metals processingindustry, in semiconductor manufacture, in petroleum desulfurization,for power generation via electrochemical fuel cells and combustionengines, and as a feedstock in ammonia synthesis and other chemicalprocesses.

Hydrogen is typically produced industrially from hydrocarbon fuels viachemical reforming using combinations of steam reforming and partialoxidation. Steam reforming of the simple hydrocarbon methane occurs viathe following reaction:CH₄+H₂O→CO+3H₂This reaction occurs in the presence of a catalyst and is highlyendothermic. The extent of the reaction is low at low temperatures. Inconventional reforming processes, a temperature as high as 800° C. isoften required to convert an acceptable amount of hydrocarbon fuel intocarbon monoxide and hydrogen.

The steam reforming catalyst typically employed in industrial reactorscontains an active Ni metal component supported on a ceramic oxidecontaining a mixture of aluminum oxide with Ca or Mg. However, O₂present in hydrocarbon fuel can cause the Ni to form nickel oxide, whichis inactive as a steam reforming catalyst. The Ni metal can also reactwith the aluminum oxide of the support to form compounds that arecatalytically inactive for steam reforming, such as nickel aluminatespinel. This detrimental interaction between active metal and supportcan significantly reduce catalyst activity over long periods ofoperation.

In some cases reforming catalyst is exposed to cyclic operationconditions of reactor shut-downs and restarts. This cyclic operation ismore important for fuel cell and small scale hydrogen generation plantsthan for conventional large scale hydrogen production plants. Duringreactor shut-down, it is desirable that exposure of catalyst to air doesnot lead to a significant loss in catalytic activity. However, exposureof Ni to air during each cycle incrementally leads to reduced catalystactivity as the Ni becomes increasingly oxidized. Under theseconditions, the oxidized nickel must be reduced if the Ni-based catalystis to regain activity.

Because O₂ may be present at relatively high levels in hydrocarbonfeeds, especially in natural gas obtained from a utility, a process forremoving O₂ from the hydrocarbon must be included upstream of thereforming reactor to avoid oxidation of the Ni metal catalyst.

An additional problem with conventional Ni-based catalyst is that the Nimetal is susceptible to poisoning and deactivation by trace levels (˜1ppm) of sulfur (S) in the reacting hydrocarbon fluid. Removal of sulfurto levels acceptable for Ni-based reforming catalysts requires ahydrodesulfurization process and a sulfur absorption bed, both of whichadd to the complexity, cost and size of the reformer system.

Alternative catalysts for steam reforming processes have been proposed.

Rostrup-Nielsen, Jens R., Catalytic Steam Reforming, Springer-Verlag,Berlin, 1984, suggests that for steam reforming Rh and Ru are the mostactive catalysts, while Pt, Ni and Pd are all comparable, and Ir is lessdesirable.

U.S. Pat. No. 4,988,661 discloses hydrocarbon steam reforming catalystshaving nickel oxide, cobalt oxide and/or platinum group noble metalssupported on a carrier consisting essentially of aluminum oxide and anoxide of Ca, Ba and/or Sr.

U.S. Pat. No. 6,238,816 discloses sulfur-tolerant catalysts forhydrocarbon steam reforming. The catalysts contain active metals of Ag,Co, Cr, Cu, Fe, Pd, Pt, Ru, Rh, and/or V supported on various oxidematerials.

While conventional hydrocarbon steam reforming catalysts provideimproved initial activity and sulfur tolerance relative to Ni-basedcatalysts, conventional catalysts fail to provide stable performanceover extended periods of time upon exposure to both air and reducingatmospheres. Conventionally, catalyst stability is measured in air.However, catalyst stability in air is no indication of catalyststability in low oxygen and reducing environments.

SUMMARY OF THE INVENTION

The present invention provides a catalyst containing an active metal,such as Ir, Pt and/or Pd, on a stable, high surface area, catalystsupport. The catalyst has improved sulfur tolerance, activity, andlong-term stability in both air and reducing atmospheres.

The active metal is resistant to sulfur and can have a free energy ofsulfide formation, ΔG°_(sulfide), less negative than about −50 kJ/mol at527° C. and less negative than about −20 kJ/mol at 727° C. The term“ΔG°_(sulfide)” as used herein refers to the free energy of sulfideformation for the reaction H₂S+xMe→H₂+Me_(x)S, where Me is the activemetal and Me_(x)S is the metal sulfide having the most negative freeenergy of formation at the reaction temperature.

The catalyst support includes at least one ceramic material, such asmonoclinic zirconia and/or an alkaline-earth metal hexaaluminate, thatretains a high surface area after exposure for 100 hours, at atemperature of about 750° C. and a pressure of about 100 psig, toatmospheres of both air and a reducing 75 vol % H₂/25 vol % H₂Oatmosphere.

Stable, high surface area, catalyst supports can be made by heattreating precursor materials in a low oxygen atmosphere, e.g., at atemperature of no more than 1100° C. in an atmosphere having an O₂partial pressure of 0.20 atm or less and containing at least 50 vol % ofat least one selected from the group consisting of H₂, H₂O and an inertgas. The resulting catalyst supports do not require binders, and can besubstantially single phase materials.

The catalyst of the present invention can be used to generate H₂ byhydrocarbon steam reforming using feedstreams contaminated withsignificantly more sulfur and oxygen than is conventionally feasible.The catalyst is tolerant of reduction-oxidation cycles. The stability ofthe support in both air and reducing atmospheres allows the active metalon the support to remain dispersed during hydrocarbon reforming. As aresult, the catalyst retains its catalytic activity significantly longerthan conventional catalysts.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of this invention will be described in detailwith reference to the following figures.

FIG. 1 shows a cross-sectional view of a catalyst pellet.

FIG. 2 shows the initial variation in activity coefficient withtemperature of catalysts containing Ir, Pt or Rh supported on a mixtureof CaO•Al₂O₃, CaO•2Al₂O₃, CaO•6Al₂O₃ and alumina in a neat hydrocarbonfeed.

FIG. 3 shows the variation in activity coefficient with temperature ofthe catalysts used to produce FIG. 2 when the catalysts were exposed toa sulfur-containing hydrocarbon feed, after the catalysts had first beenaged for 100 hours in a hydrocarbon/steam atmosphere.

FIG. 4 compares the variation in activity coefficient with temperatureof a catalyst containing 4 wt % Ir on a mixed calcium aluminate/aluminasupport, after aging for about 5 days, to that of a catalyst containing4 wt % Ir on a monoclinic zirconia support, after aging under similarconditions for about 11 days.

FIG. 5 compares x-ray diffraction patterns of fresh and aged catalystscontaining 2 wt % Ir on a mixed calcium aluminate/alumina support.

FIG. 6 compares x-ray diffraction patterns of fresh and aged catalystscontaining 2 wt % Ir on a pure monoclinic zirconia support.

FIG. 7 compares x-ray diffraction patterns of fresh and aged bariumaluminate supports.

FIG. 8 compares the variation of surface area with aging temperature foraged catalyst supports of mixed calcium aluminate/alumina, nickelaluminate, monoclinic zirconia, or barium aluminate after each of thesupports was aged in an atmosphere containing hydrogen and steam for 4days.

FIG. 9 compares the variation in activity coefficient with temperatureof a catalyst containing 1 wt % Ir on a mixed calcium aluminate/aluminasupport, after 500 hours of continuous operation using feeds containingsulfur and oxygen, to that of the same catalyst after 500 hours ofoperation using the same feeds and with air cycling.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The catalyst of the present invention includes an active metal on astable catalyst support.

In embodiments, the active metal comprises at least one of Ir, Pt andPd. Preferably, the active metal has a free energy of sulfide formation,ΔG°_(sulfide), less negative than about −50 kJ/mol at 527° C. and lessnegative than about −20 kJ/mol at 727° C. More preferably, ΔG°_(sulfide)is less negative than about −30 kJ/mol at 527° C. and greater than about0 kJ/mol at 727° C. The low affinity of the active metal for sulfurtends to make catalysts containing the active metal more tolerant ofsulfur in feeds.

Preferably, the active metal has a melting point greater than about1550° C. at a pressure of 1 atm. More preferably, the active metal has amelting temperature greater than 1750° C. at 1 atm. The relative highmelting point of the active metal helps to prevent active metaldispersed on a catalyst support from agglomerating during catalyst use.Such agglomeration can lead to a reduction in active metal surface areaand to a reduction in catalyst activity.

Table I lists ΔG°_(sulfide) and melting point for various metals. Ni,which is not a platinum group metal, has a ΔG°_(sulfide) significantlymore negative than −50 kJ/mol at 527° C. and significantly more negativethan about −20 kJ/mol at 727° C. Ru, which is a platinum group metal,has ΔG°_(sulfide) more negative than −50 kJ/mol at 527° C. and morenegative than about −20 kJ/mol at 727° C. The melting point of Ru is2310° C. Because ΔG°_(sulfide) for both Ni and Ru are so negative,catalysts based on Ni or Ru are highly susceptible to sulfurcontamination.

In contrast to Ni and Ru, Table I shows that Pd, Pt and Ir each has aΔG°_(sulfide) less negative than −50 kJ/mol at 527° C. and less negativethan about −20 kJ/mol at 727° C. Pd, Pt and Ir also have melting pointshigher than 1550° C. Thus, Ir, Pd and Pt are preferred active metals. Iris the active metal with the most preferred combination of G°_(sulfide)and melting point. TABLE I ΔG°_(sulfide) (kJ/mol at Metal Melting PointMetal/Sulfide (kJ/mol at 527° C.) 727° C.) (° C.) Ni/Ni₃S₂ −98.4 −95.31453 Ru/RuS₂ −70 −57 2310 Pd/PdS −9 −0.6 1552 Pt/PtS −16 −9 1772Ir/Ir₂S₃ −22 3 2410

The catalyst can contain from 0.01 to 6 wt %, preferably 0.1 to 4 wt %,of the active metal. The amount of active metal loaded on the catalystis tailored to the process conditions (e.g., total pressure,temperature) under which the catalyst is used and to the feedstockcomposition (e.g., sulfur activity). At higher sulfur activity and lowertemperatures the metal loading is generally increased, while at lowersulfur activity and higher temperatures the metal loading is generallydecreased. The preferred loading may also be tailored to achieve adesired reaction rate to achieve preferred heat flux profiles within areactor.

In addition to the active metal, the catalyst can contain at least oneadditional metal other than Ir, Pt and Pd. Preferably, the additionalmetal exhibits some catalytic activity. The additional metal need notmeet the ΔG°_(sulfide) and melting point criteria set forth above forthe active metal. Examples of the additional metal include Ni, Co andRu, along with other metals known in the art of steam reforming. Theaddition of small amounts of Ir, Pt and/or Pd to conventional catalystscontaining Ni, Co or Ru can reduce reaction initiation temperatures inthe presence of one or more feedstock impurities and can also facilitatecatalyst regeneration after poisoning by sulfiding, coking or oxidation.

Preferably, the active metal and any additional metal are each dispersedon the catalyst support. More preferably, each of the metals isuniformly dispersed on the catalyst support.

The catalyst support includes at least one ceramic material havingimproved stability in the low O₂ and reducing atmospheres encountered inhydrocarbon steam reforming processes.

Conventionally, catalyst supports are almost uniformly calcined in airduring manufacture. As a result, these supports are stabilized in an airenvironment (O₂ partial pressure of 0.21 atm). However, the presentinventors have found that ceramic stability in air is no guarantee ofstability in the low O₂ and reducing atmospheres encountered in steamreforming.

The present inventors have found that thermodynamic instabilities in airstabilized supports can be identified by exposing the air stabilizedsupports to low O₂ environments. The present inventors have developed anaging test for uncovering ceramic materials having improved stability inlow O₂ and reducing atmospheres.

The aging test involves exposing candidate materials for 100 hours, at atemperature of 750° C. and a pressure of 100 psig, to a 75 at % H₂/25 at% H₂O atmosphere. The test can include cycling between air and the 75 at% H₂/25 at% H₂O atmosphere. These test conditions are intended to mimicthe most aggressive oxidation and reduction/hydrothermal conditions towhich a catalyst will be exposed during steam reforming. Ceramicmaterials that pass the test can be used to form catalyst supports andcatalysts having improved stability relative to conventional steamreforming catalyst materials.

The stability of the ceramic material during the 100 hour aging test isreflected in a lack of an appreciable change in the composition of theceramic material during the test, as monitored by various diffractiontechniques (e.g., x-ray diffraction) known in the art. During the 100hour aging test, at least 80 vol %, preferably at least 90 vol %, morepreferably at least 95 vol %, of the ceramic material remains in itsoriginal crystallographic phase.

Ceramic materials that can be used in the catalyst and catalyst supportof the present invention include monoclinic zirconia (i.e., monoclinicZrO₂) and alkaline-earth metal hexaaluminates (i.e., MeO•6Al₂O₃ orMeAl₁₂O₉, where Me is an alkaline-earth metal). Alkaline-earth metalsinclude Ca, Ba, Sr and Ra. Preferably, the alkaline-earth metal in thehexaaluminate catalyst support is Ca, Ba or Sr. Barium hexaaluminate(BaO•6Al₂O₃ or BaAl₁₂O₁₉) is particularly preferred as the ceramicmaterial for the catalyst support. The catalyst and catalyst support caninclude one or more of the monoclinic zirconia and alkaline-earth metalhexaaluminates. The catalyst and catalyst support can also include oneor more ceramic materials in addition to the monoclinic zirconia andalkaline-earth metal hexaaluminates. However, preferably the catalystsupport contains at least 95 vol %, more preferably at least 98 vol %,of at least one of the monoclinic zirconia and the alkaline-earth metalhexaaluminates. Even more preferably the catalyst support is asubstantially single phase material.

After the 100 hour aging test, conventional catalyst supports havesurface areas of about 2 m²/g or less. In contrast, after the 100 houraging test the catalyst support of the present invention can have asurface area of at least 6 m²/g, preferably at least 12 m²/g, morepreferably at least 18 m^(2/)g. Surface areas can be measured by varioustechniques known in the art, for example nitrogen adsorption using theBrunauer, Emmett, and Teller (BET) technique. By retaining more surfacearea than conventional catalyst supports after the 100 hour aging test,the catalyst support of the present invention facilitates the continueddispersion and activity of the active metal on the support.

As a result of the stability of the catalyst support and the toleranceof the catalyst active metal to impurities in the hydrocarbon feed, thecatalyst of the present invention has improved stability under a broadrange of conditions. The stability of the catalyst upon long-termexposure to hydrocarbon feeds containing oxygen and sulfur compounds isreflected in a catalyst activity coefficient that, after the 100 houraging test, is at least 50%, preferably at least 60%, more preferably atleast 70%, of the activity coefficient of the catalyst before the agingtest. The term “activity coefficient” as used herein has units ofreciprocal time and refers to the ratio of the reaction rate tohydrocarbon concentration, assuming that the catalytic reaction is firstorder in the hydrocarbon concentration. In other words, assuming thereaction rate equation is r=kC, where r is the reaction rate (in unitsof, e.g., moles per second per liter) and C is the hydrocarbonconcentration (in units of, e.g., moles per liter), the activitycoefficient is k (in units of, e.g., sec⁻¹).

In preferred embodiments, the catalyst of the present inventioncomprises Ir on a monoclinic zirconia support. hi other preferredembodiments, the catalyst comprises Ir on an alkaline-earth metalhexaaluminate support; in particular, Ir on a barium hexaaluminatesupport.

The catalyst supports of the present invention can be produced byconventional techniques known in the art. For example, precursor oxidescan be mixed by ball milling and other techniques, and the mixed powdercalcined in air at temperatures in excess of 1400° C. to form thesupport. The supports can also be synthesized using wet chemicaltechniques, such as co-precipitation of metal salts dissolved insolution, freeze-drying of metal salt-solutions or precipitates, spraydrying of metal precursors, or spray pyrolysis of metal precursors,followed by calcination in air. Precursors for producing alkaline-earthmetal hexaaluminates include alkaline-earth metal oxides, hydroxides,nitrates and alkoxides; and aluminum oxide, nitrate, hydroxides andalkoxides.

Monoclinic zirconia occurs naturally as the mineral baddeleyite.Monoclinic zirconia can also be made from zircon sand by processes knownin the art. The monoclinic crystal structure provides a zirconia that isless dense than conventional stabilized zirconias having the tetragonalcrystal structure.

Alkaline-earth metal aluminate catalyst supports produced byconventional techniques have relatively low surface areas.Conventionally, alkaline-earth metal aluminate precursors are heated inatmospheric air (partial pressure O₂ of 0.21 atm) at temperatures wellabove 1100° C. to form the thermodynamically stable hexaaluminatecrystal structure. Calcination temperatures of greater than 1400° C. inair are often required to form pure BaAl₁₂O₁₉. However, these hightemperatures lead to significant amounts of sintering and densificationin conventional hexaaluminate supports. The high calcinationtemperatures result in stable, but low surface area, supports.

Surprisingly, the present inventors have found that higher surface areacatalyst supports of almost pure alkaline-earth metal hexaaluminate canbe formed by heating precursors in low O₂ atmospheres (i.e., at partialpressures of O₂ less than the 0.21 atm O₂ of air) and at relatively lowtemperatures of 1100° C. or less. In embodiments of the presentinvention the precursor material is heated at a temperature of no morethan 1100° C., preferably no more than 950° C., more preferably no morethan 800° C., in a low O₂ atmosphere having an O₂ partial pressure of0.20 atm or less, preferably 0.10 atm or less, more preferably 0.01 atmor less, and containing at least 50 vol %, preferably at least 75 vol %,more preferably at least 90 vol %, of at least one selected from thegroup consisting of H₂, H₂O and an inert gas. Inert gases includesubstantially unreactive gases such as N₂ and noble gases such as He,Ne, Ar, Kr and Xe. The heating in a low O₂ atmosphere of the presentinvention includes heating in a vacuum at a total pressure of less than1 atm. The heating in a low O₂ atmosphere of the present invention alsoincludes heating at a total pressure of greater than 1 atm. Thealkaline-earth metal hexaaluminate produced by the low temperatureprocess can have relatively stable surface areas of 6 m²/g, preferably12 m²/g, more preferably 18 m²/g. These surface areas are in excess ofthe hexaaluminate surface areas obtained using conventional calcinationtemperatures of greater than 1100° C.

As discussed above, barium hexaaluminate (i.e., BaAl₁₂O₁₉ or BaO•6Al₂O₃)is a preferred catalyst support. The barium hexaaluminate support can bemade by first preparing a barium aluminate sample by coprecipitation ofbarium and aluminum precursors in a Ba:Al molar ratio of about 1:12 froman aqueous solution. The precipitate is then dried and calcined in airat around 1100° C. The calcined barium aluminate sample is then placedin a reactor and treated with 75 vol % H₂/25 vol % H₂O at a temperatureof about 950° C. for no more than 100 hours to form a high surface areabarium hexaaluminate material.

Optionally, the catalyst support material can be pressed into tablets,can be mixed with an additional material (e.g., binder) and extruded, orcan be shaped using other techniques known to those skilled in the art.If additional materials are added during the shaping process, thecombined material can be heat-treated in an atmosphere containinghydrogen, water, an inert gas or combinations thereof, to produce ahigh-surface area material that is stable under reducing conditions.

The heat treatment in a low O₂ atmosphere of the present invention canbe conducted at different stages of the catalyst support manufacturingprocess. For instance, precursor material can be treated prior to mixingwith a binder for forming a tablet or an extrudate, Alternatively, thetreatment can be applied after the precursor is formed into a finalshape. Repeated heat treatments can also be performed. For example,after precursors are first heat treated to form a high surface area,phase-stable powder and then processed into a tablet, extrudate orwashcoat, the processed material can be heat treated a second time tostabilize the system. The heat treatment in a low O₂ atmosphere of thepresent invention can also be applied to finished catalyst supportparticles, manufactured using conventional air calcinations, before theaddition of active catalyst metal. The treatment stabilizes the surfacearea of the support particles and prevents the loss of active metalsurface area that would result from encapsulation of active metal withina collapsing support structure.

The catalyst of the present invention can be produced by introducing theactive metal of Ir, Pt and/or Pd and the optional additional metal ontothe catalyst support. The metals can be introduced onto the supportusing various methods known in the art, such as impregnation,precipitation, and deposition. For example, metals can be introducedinto a catalyst support by impregnating the support with an aqueous ororganic solution of Ir, Pt and/or Pd salts. Organometallic complexes ofIr, Pt and/or Pd can be deposited onto a support to introduce the metal.Metal salts and complexes include chlorides, nitrates, acetates,acetylacetonates and oxylates. The metal dispersion can be optimizedusing techniques known in the art. For example, the active metal can bedistributed homogeneously throughout the catalyst support pellet orparticle in order to deter ripening of active metal crystallites andsubsequent loss of active metal surface area. Alternatively, the activemetal can be concentrated near the surface of a catalyst support pelletor particle. FIG. 1 shows such an embodiment. FIG. 1 shows across-sectional view of a catalyst pellet 10, which comprises a catalystsupport inner region 1 surrounded by a catalyst support outer region 2,where the outer region 2 comprises more dispersed active metal, e.g.,Ir, (not shown) than the inner region 1.

In contrast to conventional catalysts, the catalyst of the presentinvention can be used in steam reforming processes in the presence ofsulfur and O₂ for the production of hydrogen from fuel sources such asnatural gas, liquefied petroleum gas, alcohols, naphtha, and otherhydrocarbon fuels containing one or more of methane, ethane, propane andbutane. The catalyst of the present invention is capable of operating ina hydrocarbon fuel feed containing 1 ppm by mass or more, 10 ppm by massor more, even 100 ppm by mass or more, of sulfur. The catalyst of thepresent invention is insensitive to the presence of O₂ in the feed andis capable of operating in a hydrocarbon fuel feed containing 1 ppm bymass or more of oxygen atoms other than the oxygen atoms in steam. Inembodiments, the catalyst of the present invention is capable ofoperating in a hydrocarbon fuel active feed containing 100 ppm by massor more, e.g., 0.01 to 10 vol %, preferably 1 to 10 vol %, of O₂.Because the catalyst of the present invention is tolerant of sulfur andoxygen, it can be used in steam reforming without the costlypretreatment of hydrocarbon fuel (e.g., by partial oxidation,hydrodesulfurization, adsorption, absorption, etc.) to remove sulfur andO₂ that is typically required when conventional catalysts are used. Thecatalysts of the present invention provide optimal activity forreforming systems that operate more than 250 hours and on impure feeds,such as those found in systems for the production of hydrogen fromhydrocarbon fuel such as natural gas, propane, naphtha, and otherhydrocarbons containing sulfur. In preferred embodiments, the catalystof the present invention can be used in a system for H₂ generationthrough steam reforming such as that disclosed in U.S. Pat. No.6,497,856. The disclosure of U.S. Pat. No. 6,497,856 is incorporated byreference herein in its entirety.

A system incorporating the catalyst of the present invention is capableof quicker and simpler startup from a cold or idle condition thansystems incorporating conventional catalysts. The catalyst of thepresent invention can be shut down from operation and restarted withoutthe use of reducing or inert gas. This process simplification reducesreformer system cost relative to conventional systems by eliminatingcomponents. The simplification also improves safety and durability byreducing the number of interconnections, which can develop leaks inservice.

In embodiments, the catalyst of the present invention can be used insteam reforming processes in which the catalyst is exposed one or moretimes to each of an active feedstream, which contains a gaseoushydrocarbon and steam, and an inactive feedstream, which comprises airand/or steam but less than 100 ppm by mass of the gaseous hydrocarbon.The inactive feedstream can contain 100 ppm by mass or more, e.g., 0.01to 10 vol %, preferably 1 to 10 vol %, of O₂. In embodiments, thecatalyst is exposed cyclically to the active and inactive feedstreams.

The catalysts of the present invention exhibit, relative to conventionalcatalysts, significantly improved activity and long-term stability undernormal hydrocarbon steam reforming conditions.

EXAMPLES Example 1

The activity of fresh catalysts containing Rh, Pt or Ir was comparedwith that of similar catalysts after aging.

Catalysts were prepared using a mixed calcium aluminate/alumina supportloaded with 1 wt % Rh, Pt or Ir as active metal. The catalyst wassynthesized by impregnating Rh, Pt, or Ir on a commercially availablecalcium aluminate/alumina support. The metals were deposited from anaqueous solution of the metal chloride or hexachloro-metal acid salt.After the supports were impregnated with the metal-containing solutions,the materials were dried at about 110° C. for 24 hours and then calcinedin air at about 500° C.

FIG. 2 shows the variation of activity coefficient with temperature foreach of the fresh catalysts in neat hydrocarbon feeds of methane withwater added in a steam-to-carbon (methane) ratio of about 4:1. All threefresh catalysts demonstrate activity within the temperature range ofabout 600° C. to 800° C.

FIG. 3 shows the variation of activity coefficient with temperature foreach of the catalysts in sulfur-containing hydrocarbon feeds of methanecontaining approximately 10 ppm of sulfur in the form of hydrogensulfide after the catalysts had first undergone 100 hours of aging atabout 750° C. in about 175 psig of a hydrocarbon feed of line naturalgas with water added in a steam-to-carbon ratio of about 4:1.

FIGS. 2-3 show that with hydrocarbon feed essentially uncontaminated bysulfur and/or oxygen, the fresh Rh-containing catalyst was more activethan the fresh Ir- and Pt-containing catalysts. However, after thecatalysts underwent the 100 hours aging test and were then subjected tohydrocarbon feed streams containing sulfur, the Ir-containing catalystwas more active than the Rh- and Pt-containing catalysts.

The stability of the Pt- and, in particular, Ir-containing catalystsrelative to the Rh-containing catalyst is surprising. Conventionallyactive metals with higher initial activities (e.g., Rh) are favored forreforming catalysts. However, FIGS. 2-3 indicate that active metals withlower initial activity (e.g., Pt and Ir) can provide steam reformingcatalysts with superior long-term performance, stability, and sulfurtolerance.

Example 2

The activity of catalysts containing Ir on different catalyst supportswas compared.

A catalyst was prepared by impregnating 4 wt % Ir on a mixed calciumaluminate/alumina support. The metal was deposited from an aqueoussolution of hexachloroiridic acid. After the support was impregnatedwith the metal-containing solution, the catalyst was dried at about 110°C. for 24 hours and then calcined in air at about 500° C. The catalystwas aged for about 5 days on a hydrocarbon feed containing sulfur andoxygen, at a steam-to-carbon molar ratio of about 4, and at an averagetemperature of about 750° C.

A second catalyst was prepared by impregnating 4 wt % Ir on a puremonoclinic zirconia support. The metal was deposited from an aqueoussolution of hexachloroiridic acid onto a commercially availablemonoclinic zirconia support. After the support was impregnated with themetal-containing solution, the catalyst was dried at about 110° C. for24 hours and then calcined in air at about 500° C. The second catalystwas then aged for about 11 days under conditions similar to those usedfor the mixed calcium aluminate/alumina-supported catalyst.

FIG. 4 compares the variation in activity coefficient with temperaturefor the two aged catalysts. FIG. 4 shows that the activity of thecatalyst with the pure monoclinic zirconia support was superior to thatof the catalyst with the calcium aluminate/alumina support despite thelonger aging time of the monoclinic zirconia supported catalyst.

Example 3

The stability of different catalysts upon aging was studied.

A catalyst was prepared by impregnating 2 wt % Ir on a mixed calciumaluminate/alumina support. The metal was deposited from an aqueoussolution of hexachloroiridic acid onto a commercially available calciumaluminate/aluminate support. After the support was impregnated with themetal-containing solution, the catalyst was dried at about 110° C. for24 hours and then calcined in air at about 500° C.

A second catalyst was prepared by impregnating 2 wt % Ir on a monocliniczirconia support. The metal was deposited from an aqueous solution ofhexachloroiridic acid onto a commercially available monoclinic zirconiasupport. After the support was impregnated with the metal-containingsolution, the catalyst was dried at about 110° C. for 24 hours and thencalcined in air at about 500° C.

Each catalyst was placed in a reactor and aged for five days at about750° C. in the presence of sulfur-containing hydrocarbon feeds with asteam-to-carbon ratio of about four. After aging, the catalysts wereremoved from the reactors and analyzed.

FIG. 5 includes an x-ray diffraction pattern of the catalyst containing2 wt % Ir on the mixed calcium aluminate/alumina support when thecatalyst was fresh. For comparison, FIG. 5 also includes an x-raydiffraction pattern for the same catalyst after the catalyst was aged at750° C. for 5 days in an atmosphere containing hydrogen and steam. FIG.5 shows that initially the mixed calcium aluminate/alumina supportcontained at least four phases: CaAl₂O₄, CaAl₄O₇, CaAl₁₂O₁₉, and Al₂O₃.Despite the high loading of Ir on the catalyst, Ir diffraction peakswere not discernable in the fresh sample due to the high dispersion ofthe Ir on the support. FIG. 5 also shows that after aging the phasecomposition of the support had changed to predominately CaAl₁₂O₁₉ andAl₂O₃, with some CaAl₄O₇ remaining. After aging, Ir diffraction peakswere visible due to extensive agglomeration and sintering of the activeIr metal.

FIG. 6 includes an x-ray diffraction pattern of the second catalyst,containing 2 wt % Ir on the monoclinic zirconia support, when thecatalyst was fresh. For comparison, FIG. 6 also includes an x-raydiffraction pattern for the same catalyst after the catalyst was aged at750° C. for 5 days in an atmosphere containing hydrogen and steam. FIG.6 shows that the zirconia-supported catalyst did not undergo asignificant phase change after aging. Ir diffraction peaks are notdiscernable after the 5 days of aging, indicating that the active Irmetal was still well dispersed on the surface of the support.

Example 4

The stability of a barium aluminate catalyst support was studied.

A catalyst support of barium aluminate was prepared by firstcoprecipitating barium and aluminum precursors in a Ba:Al molar ratio ofabout 1:12 from an aqueous solution. The precipitate was then dried andcalcined in air at 1100° C. for several hours. The catalyst support wasplaced in a reactor and aged in the presence of 75 vol % H₂/25 vol % H₂Oat 950° C. for 4 days.

FIG. 7 compares x-ray diffraction patterns of the fresh support with theaged support. Immediately after the calcination in air at 1100° C. thecatalyst support still contained a mixture of BaAl₂O₄ and BaAl₁₂O₁₉(BaO•6Al₂O₃ or BA6) phases. However, under the reducing/hydrothermalaging environment, the catalyst support converted to nearly 100%BaAl₁₂O₁₉. The aged BaAl₁₂O₁₉ catalyst support had a very stablespecific surface area in excess of 15 m²/g.

Example 5

The stability of different catalyst support materials was compared.

Catalyst supports were prepared from each of mixed calciumaluminate/alumina, nickel aluminate (NiAl₂O₄), monoclinic zirconia, andbarium aluminate. Calcium aluminate/alumina and monoclinic zirconiasupports were obtained from commercial sources. Nickel aluminate andbarium hexaaluminate were respectively synthesized by thecoprecipitation of nickel and aluminum precursors (in a ratio of 1:2)and barium and aluminum precursors (in a ratio of 1:12), followed bycalcinations in air at 1100° C. Each of the catalyst supports was agedin the same atmosphere containing hydrogen and steam (i.e., 75 vol % H₂/25 vol % H₂O) for 100 hours at various aging temperatures.

FIG. 8 shows how the surface area of the four aged catalyst supportsvaried with aging temperature.

The surface area of both the aged mixed calcium aluminate and the agedNiAl₂O₄ was less than 10 m²/g when the aging temperature exceeded about200° C., and dropped to as low as about 3 m²/g when the temperatureexceeded 700° C. This is consistent with the theory that mixed calciumaluminate supports and NiAl₂O₄ supports are unstable in H₂/H₂Oatmospheres and undergo significant phase changes that are accompaniedby significant loss of specific surface area.

In contrast to the aged mixed calcium aluminate and aged NiAl₂O₄supports, the aged pure monoclinic zirconia and aged bariumhexaaluminate supports had surface areas at about 750° C. in H₂/H₂Oenvironments in excess of 10 m²/g This indicates that pure monocliniczirconia supports and alkaline-earth metal hexaaluminate supports arestable in H₂/H₂O atmospheres and do not undergo significant phasechanges once formed. The high stable surface area of pure monocliniczirconia and alkaline-earth metal hexaaluminate supports maintainsactive metal dispersion even after hundreds of hours on-stream andresults in higher long-term catalyst activity.

Example 6

The ability of the catalyst of the present invention to withstand cyclicoperation (i.e., reactor operation, followed by shut-down, followed byrestart) was studied.

A catalyst was prepared by impregnating about 1 wt % Ir onto a mixedcalcium aluminate/alumina support. The metal was deposited from anaqueous solution of hexachoroiridic acid onto a commercially availablecalcium aluminate/alumina support. After the support was impregnatedwith the metal-containing solution, the catalyst was dried at about 110°C. for 24 hours and then calcined in air at about 500° C. The catalystwas divided into two approximately equal portions.

The first portion of the catalyst was placed in a reactor that operatedcontinuously at about 750° C. for over 500 hours on a hydrocarbon feedthat contained sulfur and O₂.

The second portion of the catalyst was placed in a reactor that wasstarted up and allowed to run at 750° C. for about 24 hours on the samehydrocarbon/sulfur/O₂ feed as was used for the first portion. Then thereactor was shut down and purged with air. The reactor was then reheatedto 750° C. and purged with steam prior to the next 24 hourfeedstock-charged cycle. These steps were repeated for over 500 hours oftotal operating time. The cycling conditions are representative of theenvironments to which the catalyst would be exposed with normal reactorstart-ups and shut-downs.

FIG. 9 compares, after the 500 hours tests, the variation in activitycoefficient with temperature of the first portion catalyst (operatedwithout air cycling continuously) to that of the second portion catalyst(operated with air cycling).

FIG. 9 shows that the catalyst of the present invention exhibitsacceptable catalytic activity both in a reactor operated with aircycling and in a reactor operated without air cycling.

The disclosure herein of a range of values is a disclosure of everynumerical value within that range. In addition, the disclosure herein ofa genus is a disclosure of every species within the genus (e.g., thedisclosure of the genus “noble gases” is a disclosure of every noble gasspecies, such as Ar, Kr, etc.).

While the present invention has been described with respect to specificembodiments, it is not confined to the specific details set forth, butincludes various changes and modifications that may suggest themselvesto those skilled in the art, all falling within the scope of theinvention as defined by the following claims.

1. A catalyst support comprising 95 vol % or more of an alkaline-earthmetal hexaaluminate, wherein the catalyst support has a surface area of6 m²/g or more.
 2. The catalyst support according to claim 1, whereinthe catalyst support comprises 98 vol % or more of the alkaline-earthmetal hexaaluminate.
 3. The catalyst support according to claim 1,wherein the catalyst support has a surface area of 12 m²/g or more. 4.The catalyst support according to claim 1, wherein the catalyst supporthas a surface area of 18 m²/g or more.
 5. The catalyst support accordingto claim 1, wherein the alkaline-earth metal hexaaluminate comprises atleast one alkaline-earth metal selected from the group consisting of Ca,Sr and Ba.
 6. The catalyst support according to claim 1, wherein thealkaline-earth metal hexaaluminate comprises BaO•6Al₂O₃.
 7. A method ofmaking a catalyst support, the method comprising heating at least oneprecursor oxide in an atmosphere having a partial pressure of O₂ of 0.20atm or less and containing at least 50 vol % of at least one selectedfrom the group consisting of H₂, H₂O and an inert gas; and producing thecatalyst support of claim
 1. 8. The method according to claim 7, whereinthe inert gas is selected from the group consisting of He, Ne, Ar, Kr,Xe and N₂.
 9. The method according to claim 7, wherein the atmospherecontains at least 50 vol % of N₂.
 10. The method according to claim 7,wherein the heating is performed at a total pressure of 1 atm.
 11. Themethod according to claim 7, wherein the heating is performed at a totalpressure of less than 1 atm.
 12. The method according to claim 7,wherein the heating is performed at a total pressure of greater than 1atm.
 13. The method according to claim 7, wherein the precursor oxidesare heated in an atmosphere containing a partial pressure of O₂ of 0.10atm or less.
 14. The method according to claim 7, wherein the heating isat a temperature of no more than 1100° C.
 15. The method according toclaim 7, wherein the heating is at a temperature of no more than 950° C.16. The method according to claim 7, wherein the heating is at atemperature of no more than 800° C.
 17. The method according to claim 7,wherein the at least one precursor oxide comprises a member of the groupconsisting of alkaline-earth metal oxides.
 18. The method according toclaim 7, the method further comprising heating the at least oneprecursor oxide in another atmosphere having a partial pressure of O₂greater than 0.20 atm.
 19. The method according to claim 18, wherein theother atmosphere is air; and the total pressure in the other atmosphereis 1 atm.
 20. The method according to claim 18, wherein the heating inthe atmosphere having a partial pressure of O₂ of 0.20 atm or less andthe heating in the other atmosphere having a partial pressure of O₂greater than 0.20 atm are each repeated more than once. 21-31.(canceled)
 32. A method of using a catalyst to generate H₂, the methodcomprising providing a catalyst comprising the catalyst support of claim1, and at least one of Ir, Pt and Pd on the catalyst support; passingover the catalyst an active feedstream comprising a gaseous hydrocarbonand gaseous H₂O; and reacting the gaseous hydrocarbon and the gaseousH₂O using the catalyst to produce the H₂.
 33. The method according toclaim 32, wherein the active feedstream comprises 10 ppm by mass or moreof S.
 34. The method according to claim 32, wherein the activefeedstream comprises 100 ppm by mass or more of S.
 35. The methodaccording to claim 32, wherein Ir is on the catalyst support.
 36. Themethod according to claim 32, wherein the gaseous hydrocarbon comprisesat least one selected from the group consisting of methane, ethane,propane and butane.
 37. The method according to claim 32, wherein theactive feedstream further comprises 100 ppm by mass or more of O₂. 38.The method according to claim 37, wherein the active feedstreamcomprises 10 ppm by mass or more of S.
 39. The method according to claim37, wherein the active feedstream comprises 100 ppm by mass or more ofS.
 40. The method according to claim 37, wherein Ir is on the catalystsupport.
 41. The method according to claim 37, wherein the gaseoushydrocarbon comprises at least one selected from the group consisting ofmethane, ethane, propane and butane.
 42. The method according to claim32, further comprising passing over the catalyst an inactive feedstreamcomprising at least one of air and gaseous H₂O, wherein the inactivefeedstream comprises less than 100 ppm by mass of the gaseoushydrocarbon.
 43. The method according to claim 42, wherein the inactivefeedstream comprises 100 ppm by mass or more of O₂.
 44. The methodaccording to claim 42, wherein the inactive feedstream comprises 1 vol %or more of O₂.
 45. The method according to claim 42, wherein Ir is onthe catalyst support.
 46. The method according to claim 42, wherein eachof the active feedstream and the inactive feedstream is passed over thecatalyst more than once.
 47. The method according to claim 32, whereinthe active feedstream comprises 1 ppm by mass or more of S.
 48. Acatalyst comprising the catalyst support of claim 1; and an active metalon the catalyst support, wherein the active metal is selected from thegroup consisting of Ir, Pt and Pd.
 49. The catalyst according to claim48, wherein the active metal is dispersed throughout the catalystsupport.
 50. The catalyst according to claim 48, wherein the catalystsupport comprises 95 vol % or more of barium hexaaluminate; and theactive metal comprises Ir.
 51. The catalyst according to claim 48,wherein the catalyst support comprises 95 vol % or more of strontiumhexaaluminate; and the active metal comprises Ir.